Large quantities of methane are available through natural gas and refinery processes such as catalytic cracking, hydrocracking, isomerization, or catalytic reforming. But methane has only minimal value as a chemical or petrochemical feedstock and is therefore often consumed by burning. In comparison, there is significant economic value in ethane and ethylene, which are products of the oxidative coupling of methane. A major drawback to oxidative coupling of methane processes is the low per-pass yield of product, usually less than 30 mole percent, and the high yield of carbon oxide by-products such as carbon monoxide and carbon dioxide. Most attempts to increase product yield have been through new catalyst formulations; see U.S. Pat. No. 4,734,537, U.S. Pat. No. 4,507,517, and Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed.; John Wiley & Sons: New York, Vol. 13 pp. 696-697 and 815-816. However, even with new catalysts, the typical yield of ethane and ethylene remains low.
The present invention is accepting of the yield limits with current catalysts and provides a process for the oxidative coupling of methane that compensates for the low product yield of standard catalysts by employing carbon conservation. The carbon oxide by-products of the oxidative coupling reaction are separated from the desired ethane and ethylene products and are reacted to form methane which is recycled to the oxidative coupling of methane reaction zone. There have been other attempts at improving the overall value of oxidative coupling of methane processes in spite of the low yields with several such attempts focusing on making further use of the undesired by-products. For example, U.S. Pat. No. 5,026,934 describes further processing the carbon oxide by-products to form methanol. Other attempts have focused on how to further process the ethane and ethylene produced by the oxidative coupling of methane; see U.S. Pat. No. 5,430,219. At least one reference has disclosed further processing of the ethane and ethylene products along with reacting the carbon monoxide by-product of the oxidative coupling of methane reaction and hydrogen to produce additional C.sub.2 +hydrocarbons; see U.S. Pat. No. 4,849,571.
The present invention is directed to increasing the ultimate production of ethane and ethylene from the oxidative coupling of methane through carbon conservation and recycle. Undesired carbon oxide by-products from the oxidative coupling reaction are separated from the desired ethane and ethylene products through either chemical reaction with the oxidative coupling catalyst itself or through adsorption. The separated carbon oxide by-products are contacted with a methanation catalyst to form methane which is dried and recycled to the oxidative coupling reactor. Through the carbon conservation and recycle, a greater portion of the available methane is ultimately converted to ethane and ethylene. Thus, the economics of methane oxidative coupling is favorably altered.